Moved from Maccaffertium.

I believe this is Stenonema femoratum based on the combination of having crowded crossveins below the bulla of the forewing and three dark dashes on the posterior margins of the abdominal segments.

Moved from Mayflies.

Well Charles, I suppose it's all relative, but this is actually a very old mayfly. The vast majority of their lives are spent in the nymphal aquatic lifestage. Then, the mature nymph emerges from its aquatic life as a winged but sexually immature subimago, which lifestage lasts for perhaps a day or two (on average). Following that, the subimago undergoes a final molt, becoming a winged and sexually mature imago, which lifestage lasts for perhaps a day (on average).

Your gorgeous photo is of a female subimago of family Heptageniidae, likely genus Maccaffertium, which is often referred to by the common name, Cream Cahill.